var items = document.getElementsByClassName('item');
var index = 0;
var goNextBtn = document.getElementById('goNext');
var goPreBtn = document.getElementById('goPre');

var clearActive = function () {
    for (var i = 0; i < items.length; i++) {
        items[i].className = 'item';
    }
}

var goIndex = function () {
    clearActive();
    items[index].className = 'item active';
}

var goPre = function () {
    if (index == 0) {
        index = 4;
    }
    else 
    index --;
    goIndex();
}
    
var goNext = function() {
    if (index < 4) {
        index ++ ;
    }
    else 
    index = 0 ;
    goIndex();
}

goPreBtn.addEventListener('click',function () {
    goPre();
})
goNextBtn.addEventListener('click',function () {
    goNext();
})